Here is my guide on adjusting the manual and autoplunger, which is really just a compilation of other posts that have been written on the subject. I'm a new pinsider, but...I did spend at least 6 hours trying to get 90%+ accuracy on both manual and auto, which I have now done,...so, I have at least some experience on the plunger. I'm also writing this post to act as a reference for myself in case I start losing accuracy in the future.
Step 1: You should make sure the prongs of the auto plunger are hitting both sides of the pinball. You can use needle nose pliers to bend them. Manually push the auto plunger forward to ensure that at full stroke, both sides are still being hit.
Step 2: Start with having the manual plunger hitting the pinball dead center.
When I did this...I first tried to keep the bottom two bolts in the same position instead of creating new holes. So, I cut some strips from a credit card and used them as shims on the outside of the plunger. Which...actually does totally work but later I found I wasn't the ideal way to adjust the manual plunger.
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So I went ahead and removed the two bottom bolts. The bolt on the left was stripped anyway, which..was obvious once I fully removed the plunger....from the factory the bolt was barely "eating" any of the wood.
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They don't really seem necessary anyway, especially in home use where the cab won't be as abused as out in the wild. The 3 other screws seem to keep the plunger in place plenty fine.
Step 3: Make sure the pinballs are not hitting the upper right flipper.
After I did step 1 and 2...I went to about 0% accuracy. I kept hitting the lower post/wall (below the upper left flipper). However, if I held the upper right flipper up, the balls would hit the upper left flipper. And, I noticed that the upper right flipper would "wobble" after each pinball would pass it. It was obviously being hit and then being redirected to the lower post on the opposite side of the PF.
First...I went ahead removed the right side plastics. Not that hard...maybe 6 screws/nuts. It makes the most sense to me, to have the screw holding the guide wall be all the way forward. That way, if balls hit the guide wall, you don't have to worry about the metal guide moving under the screw when you don't want it to. And for future adjustments, you won't have to take the plastics off.
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Its much easier/quicker to bend the end of the ball guide wall. I inserted a flat head screw driver between the post and guide wall and used a tiny bit of leverage to push the guide wall metal away from the post. Super easy to make adjustments later...I can either use my fingers to push the wall back towards the post, or use the screw driver to push it away. No removing plastics.
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Edit: Some users also found that adding a washer under the guide wall attachment tab seemed to help. Basically, the screw can possibly torque the guide wall enough where its not perpendicular with the playfield. If you are noticing that your guide wall is not perpendicular with the playfield, add a washer under the attachment tab at this time.
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Step 4: You need both the auto and manual plungers to send the balls on the same path.
Here is where it starts getting fun. You can't just start bending the guide wall to hope everything works. That's step 5. In my case...the manual plunger was hitting the bottom post, and the auto was hitting the top left flipper. Since bending the guide wall affects both equally, moving the guide wall away from the post would make the auto plunger work, but would make the manual even worse and vice versa.
Fortunately there are adjusters for both the auto and manual plungers.
Now...counter-intuitively (at least for me), increasing power makes the ball path move towards the bottom post and decreasing moves it towards the top left flipper. I would assume...that decreasing power would make it move "downward" due to gravity...but...such is not the case.
Auto plunger, after fixing the prongs in step 1 which would increase power (ball path moving downwards), you have:
Increase power: You could set your coil strength higher (like to hard). I would avoid this if at all possible because that also affects everything else (like flipper strength, popbumpers, slings, etc).
Decrease power: Easy....you just turn down the auto plunger strength setting (for example, moving it to 225 instead of 255). You can't decrease it too much because once you get past a certain point in power, a decrease in power really does move the ball in a downward path due to gravity (or not making it around the loop).
Manual plunger
Increase power: Setting the manual plunger to hit dead center on the pinball. If it already is...then you would have to switch the spring to a stronger colored spring.
Decrease power: Easy....move the auto plunger so its not quite centered. I would recommend that you keep the left/right alignment and move the plunger vertically to decrease power (if the plunger hits off centered horizontally it might slightly move the ball out of the ball guide in the lane). Its also possible maybe you could add a small washer to the plunger so it doesn't hit the ball quite as hard.
In my example....my manual ball path was lower than the auto (manual hitting post and auto hitting top flipper). So, I left the auto alone and decreased power to my manual. It was enough to get them close to the same path.
Step 5: Move the guide wall
Once both plungers have the same ball path, the easy part is then moving the guide wall so the path doesn't hit either the top left flipper or the bottom post. I would recommend trying to get the path closer to the bottom post. That way, for the manual if you don't plunge at full strength (like going for skill shot), the ball path will move upward so you want to leave some "space" so it doesn't hit the flipper. And for the auto...I would assume during the game after its used frequently (IMDN does have a lot of auto balls, revives, etc), the spring will warm up and thus lose power as well.
Now, if your ball path can't be quite exactly the same between manual/auto...hopefully they are close enough that you can still be within "tolerance" where you can move the guide wall so both paths don't hit corresponding obstacles (post/flipper).
Step 6: Add spacer to the guide wall (still under research)
I noticed that after adjustment, it doesn't take much for the alignment to get off again. I check the guide wall, and I see that its already pushed back enough to cause misalignment. With the guide wall (step 5), you are doing micro adjustments, so I can definitely understand how it quickly misaligns...the pinballs are acting like small hammer strikes every time they hit it.
So, currently I'm trying to find the best solution to the problem, which at the moment is inserting a small rubber spacer.
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I took a rubber cabinet door stopper, which was initially too big, and slowly sanded it down. It only took about 10 minutes of trial and error. I would push the wall back towards the post, insert the spacer, and test. If it was too big still (i.e., hitting the left post), I would take it out, sand it down, push the wall back in, insert spacer and retest. I did it slowly because if you sand it too much, you have to start over from the beginning. You can take away material, but you can't put it back! I'll edit this step if I find a better/easier to adjust spacer.
Step 7: Maintenance
Now, what about leveling the PF, or adjusting the pitch, or excessive nudging and now accuracy drops? Hopefully the auto/manual ball paths are still roughly the same, so you would just need to do step 5 again. If they aren't...have fun going through the steps again! I would assume that its mainly going to be step 5 maintenance...especially since after balls keep hitting the guide wall it could change the shape.
Final Notes:
To me, if you can only get 1 plunger to work correctly, I would do the auto-plunger. On a game, you are ever only doing the manual plunger 3 times, but the auto plunger gets used much more than that (well, as long as you don't have an awful game). And...its easier to do adjustments on the manual plunger "on the fly". I.e., if you are hitting the left post, then just don't do full power plunges. Hitting flipper? Try to only do full power plunges (or hold the left flipper up).
Anyway...hope you enjoy the guide and let me know if I should edit/add anything!